This invention relates to frequency synthesizers for radio frequency, microwave or gigahertz applications in which spurious frequencies are to be reduced or eliminated. Previously frequency synthesizers operating in the microwave or gigahertz range have faced a compromise between frequency range resolution and phase noise. This invention relates to a frequency synthesizer which may operate over a wide frequency range at high resolution and variable step size without trading off between spurious frequency rejection and phase noise suppression.
A frequency synthesizer is a circuit for generating a desired output frequency from one or more input frequencies. Typically, the output frequency is stabilized and phase locked to a reference frequency. In the past the synthesizer designer has needed to make comprises trading-off one area of performance for another. For example, in designing a direct digital synthesizer the process of converting the digital signals to analog waveforms causes spurious outputs. The magnitude of these spurious outputs relative to the carrier frequency increases rapidly as the output frequency increases, limiting the usefulness of such synthesizers for the gigahertz frequency range. When such a direct digital synthesizer includes a phase locked oscillator, an undesirable tradeoff between spurious outputs and phase noise often results.
Direct synthesizers achieve fast switching speeds by using combinations of arithmetic functions such as division, multiplication and mixing of a stable signal to achieve a desired output frequency. Such synthesizers, however, have been unable to achieve such performance with a small-sized system or with low phase noise and low spurious outputs. In addition, such synthesizers may suffer from high power consumption and poor frequency resolution.
Conventional synthesizers used for telecommunications or satellite communications typically are tunable over a very narrow frequency band. The architecture for such synthesizers is based on a VHF synthesizer/multiplier/upconverter approach. A varactor tuned oscillator often is used in which the oscillator Q and phase noise are inversely dependent on the tuning range. Phase noise is compromised limiting the synthesizers to a narrow operating range.
Accordingly, conventional synthesizers often are limited to a narrow frequency range over which spurious frequencies are sufficiently rejected. Tuning such a synthesizer to operate outside a select range results in unsatisfactory spurious output performance. Typically, hardware changes (system design changes) are needed to alter the operating range. Changes to the step size for tuning the oscillator also may require hardware design changes, resulting in recurring engineering charges.
FIG. 1 shows a PLL implementation of a synthesizer 10, in which a reference signal F.sub.1 and a programmed divider value N are input and a frequency-referenced, phase-referenced signal F.sub.0 is output. The synthesizer 10 includes a tuned oscillator 12, a power splitter 14, a buffer 16, a programmable frequency divider 18, a phase detector 20, a loop amplifier 22 and a loop filter 24. The frequency of the output signal F.sub.0 equals the programmed divider count, N, times the frequency of the reference signal F.sub.1. Such an implementation suffers from a trade-off between output frequency step size and phase noise, along with a limited maximum output frequency.
For higher frequency implementations, such as for a microwave frequency synthesizer 10' as shown in FIG. 2, a second source 26 often is included in the feedback path. Also added are a mixer element 28, a loop filter 30 and another buffer 32. The second source 26 generates a signal F.sub.2 used for mixing the output signal F.sub.0 so as to provide frequency increments between harmonics of the reference signal F.sub.1. The second source 26 may be another phase locked loop oscillator generating a select reference frequency, or may be a fixed reference signal source, or may be a switched reference signal source.
Use of a harmonic mixer (e.g., sampler) for the mixer element 28 also is known. A harmonic mixer reduces the required frequency F.sub.2, enabling such frequency to be in the VHF range. In addition, a narrower tuning range of the source 26 occurs, because the operating harmonic of the mixer can be incremented for large frequency steps.
In embodiments which employ mixing of multiple reference signals (i.e., F.sub.1 and F.sub.2), the resulting mixing product terms include not only the desired term, but product terms between the harmonics of F.sub.1 and F.sub.2. Such additional terms are "spurious frequencies". Such spurious frequencies may occur at very small offsets from the frequency of the desired term. Filtering such spurious frequencies requires a filter passband which is too narrow for a practical high frequency synthesizer. In addition, the step size between potential output signal frequencies would need to be unreasonably large to allow such narrow passbands.
As a result, the conventional way of reducing spurious frequency effects has been to reduce the drive of the mixer. Such an approach, however, adversely impacts the signal to noise ratio and the phase noise "floor" of the feedback loop. Thus, conventional high frequency synthesizer designs have been subject to a tradeoff between phase noise and spurious frequency performance.
Accordingly, there is a need for a high frequency synthesizer which improves both spurious frequency rejection and phase noise suppression. Further, there is a need for a synthesizer which can achieve such performance without sacrificing step size, physical size, weight or power consumption.